If something costs $1.00 and is 10% off you just move the decimal point over, so it would be .10, or 10 cents off. Therefore it costs 90 cents on sale. Ask your math teacher to help you figure this out, it becomes quite easy after you understand the mathematical principles involved.
Discounts are often calculated using percentage. For instance, if there is a 5% discount, that means you are paying 95% (100% minus 5%) of the original price. By the same principle, when there is a 50% off, you are paying half of the original price.